Process for preparing solid melamine

ABSTRACT

The invention relates to a method for producing solid melamine by expanding liquid, ammoniacal melamine which is mixed with excess ammonia, whereby a dispersion is produced. Said dispersion is expanded, optionally after it has been held under ammonia pressure, whereby solid melamine is deposited. The solid melamine is optionally held under ammonia pressure. Expansion is then optionally carried out in any particular order, along with cooling to room temperature and the pure melamine is isolated.

The invention relates to a process for preparing solid melamine, bydepressurizing a dispersion of ammonia and liquid melamine, whereuponsolid melamine precipitates.

Melamine is preferably prepared by pyrolyzing urea, using eitherlow-pressure processes or high-pressure processes, for example thosedescribed in “Ullmann's Encyclopedia of Industrial Chemistry, Vol A 16,5th ed (1990), pp. 171-185”. Depending on the preparation process, themelamine synthesized comprises from about 94 to 98% by weight ofmelamine, and also in particular melam, melem, ureidomelamine, ammeline,and ammelide as significant impurities, and has to be further purifiedthrough particular steps of the process for the more demandingapplication sectors. To obtain solid melamine, the liquid melamine meltmay be cooled, for example using water, using aqueousmelamine-containing solutions or suspensions, or using cold inert solidsor solid melamine, as in AT 159/98, for example in a fluidized bed. Aparticularly advantageous method is to inject and depressurize anammonia-containing melamine melt, for example as in WO97/20826, into acooling vessel, in which an atmosphere of ammonia is present, whereuponpure solid melamine precipitates. However, this process does not giveideal results under all conditions of pressure and temperature.

The object was therefore to find a process which, irrespective of thetemperature used and of the pressure used, gives solid melamine of goodquality via depressurization, and in a wide range of temperature and ofpressure, and in particular at low melt pressures.

It has now been found that this object can be achieved in that themelamine melt to be depressurized comprises excess ammonia in additionto the dissolved ammonia, giving a two-phase mixture in the form of adispersion of ammonia and liquid melamine.

The invention therefore provides a process for preparing solid melamineby depressurizing liquid, ammonia-containing melamine, characterized inthat

a) the liquid ammonia-containing melamine is mixed with excess ammonia,whereupon a dispersion of ammonia and liquid melamine forms,

b) where appropriate, the dispersion is aged under the pressuregenerated by ammonia,

c) the dispersion is depressurized, whereupon solid melamineprecipitates,

d) where appropriate, the solid melamine is aged under the pressuregenerated by ammonia,

e) and then, where appropriate and in any desired sequence, there isfurther depressurization to atmospheric pressure, and cooling to roomtemperature, and the melamine is isolated.

The ammonia (gas phase) is preferably supercritical and is preferably infinely dispersed form in the liquid melamine melt (liquid phase)producing a very finely dispersed “melamine foam”. The mixing processforms a dispersion of melamine and ammonia, the liquid melamine becomingsaturated with ammonia. It is preferable for the liquid melamine phaseto have been saturated with ammonia. According to the invention it ispossible either for the ammonia to have been dispersed in the liquidmelamine or for the liquid melamine to have been dispersed in theammonia. It is important that the entire amount of ammonia (dissolvedammonia and ammonia present in the gas phase) is sufficiently great forthe amount of heat dissipated during the depressurization to be thatneeded to solidify the melamine. A particular advantage of the inventionis therefore that with the aid of the excess ammonia dispersed in themelt it is possible to obtain sufficient dissipation of heat to solidifythe melamine during the depressurization even at relatively lowpressures on, and relatively high temperatures of, melamine melts whererelatively little ammonia has been dissolved in the melamine melt. Theamount of excess ammonia in the melamine melt depends in particular onthe level of temperature of, pressure on, and ammonia saturation of themelt prior to the depressurization, and on the extent to which themelamine is to be cooled below its melting point once it has beensolidified. High temperatures of, and low pressures on, the melttherefore require larger amounts of excess ammonia than temperatureswhich are just above the melting point of the melamine, which depends onthe ammonia pressure used. On the other hand, high pressures necessitatesmaller amounts of excess ammonia. The amount of excess ammonia maytherefore vary within wide limits.

Since the melting point of the melamine at lower pressures afterdepressurization is higher than at high pressures, it is also possibleaccording to the invention—in particular if the temperature of the meltis not too far above the melting point, which depends on the pressure,and the amount of excess ammonia is not very great—that the temperatureduring solidification remains the same or even rises.

According to the invention, it is preferable that the pressures at whichthe liquid ammonia-containing melamine is mixed with ammonia are fromabout 50 to 1000 bar, and that the liquid ammonia-containing melamine isthen depressurized to a pressure of from about 1 to 200 bar, whereuponsolid melamine precipitates. Depending on the procedure selected, thepressure both prior to and after the depressurization may vary over awide range. The upper pressure limit prior to the depressurization ispreferably about 600 bar, preferably about 350 bar or about 250 bar.However, the upper limit may also be about 150 bar or about 130 bar. Thelower pressure limit prior to the depressurization is preferably fromabout 60 to 80 bar. The pressure after the depressurization may likewisevary within a wide range. If an annealing process follows immediately,depressurization takes place to relatively high pressures, otherwisedepressurization to atmospheric pressure is possible. The pressure afterthe depressurization is therefore preferably from about 1 to 100 or 150bar, particularly preferably from about 1 to 60 bar. However, it mayalso be from about 10 to 20 bar.

During the mixing with ammonia, or prior to the depressurization, thetemperature of the liquid ammonia-containing melamine is preferably inthe range from about 60° C. above the melting point of the melamine,which depends on the ammonia pressure used, to just above the meltingpoint of the melamine, which depends on the ammonia pressure used,particularly preferably at temperatures between about 1 and 40° C., veryparticularly preferably between 1 and 20° C., above the melting point ofthe melamine, which depends on the ammonia pressure used. The mostuseful temperature is only very slightly above the melting point of themelamine, which depends on the ammonia pressure used. The desireddepressurization temperature is most particularly preferably below about350° C. It is preferable for the liquid, ammonia-containing melamine tohave been saturated with ammonia.

Suitable mixing equipment can be used for mixing the liquid,ammonia-containing melamine with excess ammonia, forming a dispersion,for example mixers, stirrers, reactors with naturally aspiratedstirrers, or static mixers, injectors, ejectors, or other suitablemixing equipment. The melamine melt may be mixed with either gaseous orliquid ammonia, but if liquid ammonia is used the melt must notsolidify. The temperature of the melt preferably lowers during thisprocess and is thus brought into the vicinity of the desiredtemperature, or to the desired temperature.

Prior to the depressurization, either the pressure or the temperaturemay, where appropriate, be increased, lowered or held constant, usingany desired method, either prior to or else after the mixing of melaminewith ammonia, but the melt must not solidify during this process. Toachieve a particularly good quality of melamine, it is advantageous forthe liquid melamine melt to be aged under the pressure generated byammonia prior to the depressurization, either prior to or after themixing of the liquid melamine and ammonia. This preferably takes placefor from about 1 min to 10 h depending on the process conditionsselected in the temperature range from about 350° C. to just above themelting point of the melamine, which depends on the ammonia pressureused, and preferably at temperatures above the melting point of themelamine, which depends on the ammonia pressure used, by from about 1 to60° C., particularly preferably by from about 1 to 40° C., moreparticularly preferably by from about 1 to 20° C. It is advantageoushere to lower the temperature of the liquid melamine, for example byintroducing liquid or gaseous ammonia. The pressure during the agingprocess here is in the range from about 50 to 1000 bar, preferably fromabout 80 to 600 bar, particularly preferably from about 130 to 400 bar.If the aging takes place after the mixing of melamine and ammonia, caremust be taken that the dispersion is retained during the aging process.

The temperature at which the dispersion of ammonia and liquid melamineis depressurized is preferably above the melting point of the melamine,which depends on the ammonia pressure used, by from about 1 to 60° C.,particularly preferably from about 1 to 40° C., more particularlypreferably by from about 1 to 20° C., whereupon the solid melaminedeposits, its melting point now being higher at lower pressure.

In a first embodiment of the invention, the depressurization of themelamine-ammonia dispersion follows the mixing process. Additionalammonia may also be introduced at this stage. It is preferable for thedispersion to be depressurized into a separate, and where appropriateheated, container, in which an atmosphere of ammonia is present. Themelamine dispersion here may be sprayed into the container via nozzles,e.g. single-fluid nozzles, twin-fluid nozzles or venturi nozzles, or viainjectors or ejectors, for example.

In another embodiment of the invention, it is also possible for theliquid melamine or a dispersion of melamine and ammonia to be mixed withthe excess ammonia during depressurization. This preferably takes placein specific mixing and conveying apparatuses, for example in injectorsor ejectors, in which the ammonia, as propellant gas, mixes with theliquid melamine, or liquid melamine, as propellant medium, mixes withammonia, and the resultant dispersion is simultaneously conveyed, withpressure drop, into the depressurization container.

Following the depressurization, it is also possible for the solidmelamine to be agitated, for example by stirring or by depressurizingthe melamine melt into a rotating drum or into a fluidized bed, forexample, with solidification.

The temperature of the melamine may either become lower or becomehigher, or else remain the same through the depressurization. Especiallywhen using a relatively large excess of ammonia, the temperature of thesolid melamine after depressurization is mostly lower than thetemperature of the dispersion prior to the depressurization. However,according to the invention and due to the relatively high melting pointof melamine at relatively low pressures, it is also possible andparticularly advantageous for the temperature of the solid melamineafter the depressurization to remain the same or even to rise, due tothe heat of crystallization liberated.

It has also proven advantageous for the melamine also to be aged afterthe solidification process, under the pressure generated by ammonia.During this process, after the depressurization, the solid melamine isaged in an atmosphere of ammonia, where appropriate with mechanicalstirring or in a rotating drum or through pneumatic agitation, forexample in a fluidized bed, for example for from about 10 sec to 20 h,preferably for from about 1 min to 2 h, in the temperature range fromabout 150° C. to the melting point of the melamine, which depends on theammonia pressure used. The residence time in the annealing process maybe come shorter as the temperature and pressure become higher. Thistemperature is preferably very slightly below the melting point, whichdepends on the ammonia pressure used, advantageously by up to 10° C.,particularly preferably by up to 5° C. The pressure after thedepressurization may vary over a wide range. If annealing followsdirectly, depressurization takes place to a relatively high pressure,but otherwise depressurization to atmospheric pressure is possible. Thepressure after the depressurization is therefore preferably from about 1to 150 or 100 bar, particularly preferably from about 1 to 60 bar.However, it may also be from about 10 to 20 bar. If the temperatureduring the depressurization and solidification remains the same orrises, it is advantageous for the solid melamine forming during thisprocess from the melamine melt to be aged (annealed) under theconditions of temperature and of pressure at which it precipitates afterthe depressurization.

The process of the invention can give melamine of purity above 99% byweight. Depending on the conditions of temperature and of pressureselected prior to and during the depressurization, it is also possibleto obtain melamine with a purity of up to 99.9% by weight, in some casesabove 99.99% by weight, and it is particularly advantageous here to usehigh ammonia pressures, and also temperatures near to the melting pointof the melamine.

The process of the invention may be carried out either batchwise orcontinuously. The process is preferably suitable to follow a process forpreparing melamine, in particular to follow any desired high-pressureprocess for preparing melamine from urea in which the melamine is firstproduced in liquid form, as a melt. High-pressure processes usually givemelamine in liquid form, as a melt, at pressures of from about 70 to 800bar and at temperatures—depending on the pressure selected—of at leastabout 360° C. The exhaust gases arising during melamine synthesis, inparticular NH₃, CO₂, and gaseous melamine, are usually scrubbed by beingpassed through a urea melt. During this process, the urea melt is heatedby the hot exhaust gases and is advantageously passed into a melaminereactor for melamine synthesis, while the purified exhaust gases areadvantageously passed into a urea reactor. The exhaust gases may eitherbe directly passed into the urea reactor or are condensed, for examplewith the aid of ammonium carbonate solutions or ammonium carbamatesolutions, for example those produced in the melamine plant or in theurea plant. One way of using the heat arising is for preheating theammonia used in the urea plant, or for producing steam. Once the exhaustgases have been removed, the melamine melt may advantageously bestripped, for example using NH₃, this process primarily removingresidual CO₂. It is moreover advantageous to age the melamine melt in anaging container. However, it is also possible for the melamine meltemerging from the reactor to be mixed directly with excess ammonia oncethe exhaust gases have been removed.

The advantage of the process of the invention is primarily that,whatever the degree of saturation of the melamine melt with dissolvedammonia, this depending on the pressure used and on the temperatureused, it is possible, in addition, to introduce a variable amount ofammonia into the melamine melt. This permits simple control, over a widerange, of the temperature used during the depressurization of themelamine melt, as required by process-related and product-relatedrequirements, and depending on the amount of excess ammonia in the melt.The process also permits, for example, solid melamine of good quality tobe produced, even at low pressures. A further advantage is theversatility to produce melamine in the purity required for the relevantapplications. If no cooling takes place during the solidificationprocess, there is the additional advantage that any annealing carriedout requires no, or only little, introduction of additional heat.

What is claimed is:
 1. A process for preparing solid melamine bydepressurizing liquid ammonia-containing melamine, wherein: a) theliquid ammonia-containing melamine is mixed with excess ammonia,whereupon a dispersion of gaseous ammonia and liquid melamine forms, b)optionally, the dispersion is aged under the pressure generated byammonia, c) the dispersion is depressurized, whereupon solid melamineprecipitates, d) optionally, the solid melamine is aged under thepressure generated by ammonia, e) and then, optionally and in anydesired sequence, there is further depressurization to atmosphericpressure, and cooling to room temperature, and the melamine is isolated.2. The process as claimed in claim 1, wherein the liquidammonia-containing melamine is mixed with excess ammonia andsimultaneously depressurized, whereupon solid melamine precipitates. 3.The process as claimed in claim 2, wherein the mixing of the liquidammonia-containing melamine with excess ammonia takes place withsimultaneous depressurization with the aid of injectors or of ejectors.4. The process as claimed in claim 1, wherein solid melamineprecipitates during depressurization at the same temperature, or with anincrease in the temperature.
 5. The process as claimed in claim 1,wherein the liquid ammonia-containing melamine has been saturated withammonia.
 6. The process as claimed in claim 1, wherein the pressure atwhich the liquid ammonia-containing melamine is mixed with excessammonia is from about 60 to 600 bar and the liquid ammonia-containingmelamine is then depressurized to a pressure of from about 1 to 60 bar,whereupon solid melamine precipitates.
 7. The process as claimed inclaim 6, wherein the pressure at which the liquid ammonia-containingmelamine is mixed with excess ammonia is from about 80 to 350 bar, andthe liquid ammonia-containing melamine is then depressurized to apressure of from about 1 to 20 bar, whereupon solid melamineprecipitates.
 8. The process as claimed in claim 1 wherein thetemperature range within which the liquid ammonia-containing melamine ismixed with excess ammonia from about 60° C. above the melting point ofthe melamine, which depends on the ammonia pressure used, to just abovethe melting point of the melamine, which depends on the ammonia pressureused.
 9. The process as claimed in claim 8, wherein the temperaturerange within which the liquid ammonia containing melamine is mixed withexcess ammonia is from about 1 to about 20° C. above the melting pointof the melamine, which depends on the ammonia pressure used.
 10. Theprocess as claimed in claim 1, wherein it is downstream of a continuoushigh-pressure process for preparing melamine from urea.
 11. The processas claimed in claim 1, wherein the depressurization takes place in aseparate container in which an atmosphere of ammonia is present.
 12. Theprocess as claimed in claim 1, wherein additional ammonia is introducedduring the depressurization.
 13. The process as claimed in claim 1,wherein the liquid ammonia-containing melamine is aged under thepressure generated by ammonia prior to the depressurization.
 14. Theprocess as claimed in claim 13, wherein the liquid ammonia-containingmelamine is aged under the pressure generated by ammonia prior to thedepressurization for about 1 minute to 10 hours at a pressure which isfrom about to 1 to 20° C. above the melting point of the melamine, whichdepends on the ammonia pressure used.
 15. The process as claimed inclaim 1, wherein the dispersion of ammonia and liquid melamine isdepressurized at a temperature which is from about 1 to 20° C. above themelting point of the melamine, which depends on the ammonia pressureused.
 16. The process as claimed in claim 1, wherein after thedepressurization, the solid melamine is aged in an atmosphere of ammoniafor from about 1 minute to 2 hours, in a temperature range from about150° C. to the melting point of the melamine, which depends on theammonia pressure used.
 17. The process as claimed in claim 1, whereinthe mixing of the liquid ammonia-containing melamine with excessammonia, forming a dispersion, takes place with the aid of stirrers,static mixers, injectors or ejectors.
 18. The process claimed in claim1, wherein in step a) a dispersion of gaseous ammonia in liquid melamineis formed.
 19. The process as claimed in claim 1, wherein in step a) adispersion of liquid melamine in gaseous ammonia is formed.